Pull-off adhesion testing of paint, varnish and other coatings and films with
the PAT adhesion tester and the DFD® method

The Test Method

ASTM D4541 and ISO 4624 (EN 24624) both define the method and procedures for carrying out
pull-off adhesion testing of paints, varnishes and other coatings.

The purpose of this
test is to measure the mechanical tensile strength of a coating. The
sample will be subjected to increasing tensile stresses until the weakest path
trough the material fractures. The weakest path could be along an
interface between two coatings, a cohesive fracture within one coating, a
cohesive fracture of the substrate (e.g. concrete) or a combination of
these.

Pull-off adhesion
testing is simply like any other tensile testing but where the sample is very
short, often only a few microns, even nanometers (see illustration below).

ASTM D4541 /ISO
4624 key issues:

Loading fixtures (or test elements or dollies)

The dollies must be
cleaned sufficiently to prevent "glue failure" during testing.

Rate of Pull Force Increase

Adhesion testing to
ASTM D4541 is very much like conventional tensile testing, the only
difference is that the test sample for ASTM D4541 is very short.

"
Perpendicular pull direction"

The word
"perpendicular" does not have a proper meaning when testing the
coatings adhesion on curved surfaces. What ASTM D4541 and ISO 4624 are
really trying to say is that the pull stress must always be evenly distributed
in the tested coating.

Self-leveling pull
testing devices, etc.: Beware!

The only thing that
matters for accurate adhesion test results is that the pull stress is 100%
evenly distributed throughout the pulled coating. If not, the area where
the stress is concentrated will fracture long before maximum stress has been
reached elsewhere. Result: Test results will be far too low.

Gage accuracy

The gage or the
result display of the test equipment must be calibrated to acceptable norms.

Adhesive

Use a "non-aggressive"
adhesive which does not alter the coating chemically thereby weakening it.

If
you want to be absolutely sure that you:

never reject or
question perfectly good coatings

can win
contract by demonstrating that you have full control over the coating
quality